1. Field of the Invention
This invention relates to the field of satellites, and in particular to the field of satellite maintenance and diagnosis.
2. Description of Related Art
The information age produces a continuing demand for communications. Satellite systems are often the system of choice for communications, because of their wide range and virtually unobstructed field of view of large areas of the earth. Satellites are also used for gathering information that is difficult or impossible to obtain from earth based stations.
Due to the high cost of launching and deploying a satellite, communications and information gathering satellites are typically designed as high capacity or multi-function systems, to defray the cost among multiple users or multiple applications. Also because of the high cost of replacing a satellite, most satellites are designed to be highly reliable and highly robust. Programmable on-board systems are utilized to allow for changes of function or configuration in the event of a fault; redundant components and capabilities are also often provided.
A necessary adjunct to systems that can be remotely repaired or reconfigured is a means for diagnosing the system to determine whether a repair or reconfiguration is warranted, and the particular actions necessary to effect the repair or reconfiguration. In a typical satellite system, the satellite contains signal and status monitoring equipment, and processing capabilities to provide diagnostic information to a ground station based on the monitored signals and status information. A problem with an onboard monitoring system, however, is the fact that it is on board. Although most onboard systems are designed to be independent of the mission-oriented systems that they are intended to monitor, the cost of providing an additional completely independent system from the mission-oriented system makes such an approach impractical. Other problems are also inherent to the onboard location of the diagnostic system. The proximity of the monitoring equipment to the equipment being monitored, and other equipment on board, makes it susceptible to electromagnetic interference or parasitic effects that may limit or obscure the available diagnostic information. The on board location also places the monitoring equipment "behind" the antennas used by the mission-oriented system to communicate with earth based equipment, and thus any problems related to the antenna systems cannot be monitored. This deficiency is particularly problematic for transmission systems, because the higher power devices used at the final transmission stages typically have a significantly higher failure rate than low power devices. Measurements of a satellite's antenna and transmission system are typically obtained by earth-based diagnostic systems. The accuracy, resolution, and other capabilities of an earth based system are limited, however, because of the signal degradation and interference associated with the hundreds or thousands of miles separating the earth-based systems and the satellite.